English speakers like to call this water “toilet-to-tap water.” Word for word: water passing from the bowl of your toilet to your faucet. It’s an unappealing phrase that ignores the many chemical, physical, and biological processes that recycle wastewater into high-quality drinking water.
While this idea still makes many people grimace, it is becoming a serious consideration for more and more countries and municipalities around the world that are rightfully concerned about the depletion of freshwater resources. The megacity of Bangalore in India is working on wastewater recycling, as is Los Angeles. In the United States, the Texan cities of Big Springs and Wichita Falls are already practicing this technique called direct potable reuse, as is the city of Beaufort in South Africa since 2011.
Namibia has been purifying wastewater since 1968
But in this matter, and it’s still very little known, another country remains the undisputed pioneer: Namibia. Ranked 139th on the Human Development Index, one might be surprised that this Southern African state is so advanced. However, when considering the scarce water resources in its capital, Windhoek, it becomes less surprising that this municipality sought to innovate.
In the middle of an arid plain, over 200 km from the coast, Windhoek (with nearly 500,000 inhabitants) cannot collect the scarce rainfall due to immediate evaporation in its desert-like latitudes. It cannot draw from rivers or groundwater sources in the vicinity either, as they recharge very little when they are not dried up.
In 1968, when the city, then under South African rule, saw its population growing at an impressive rate, it started recycling its wastewater into drinking water. Fifty-five years later, 30% of wastewater is recycled into drinking water in less than ten hours. The rest of the domestic drinking water comes from dams and wells in other parts of the country.
10 steps to purify wastewater
To enable the recycling of wastewater into drinking water, Windhoek has implemented a sequence of unprecedented processes, which now consists of 10 steps. These include physico-chemical processes such as coagulation and flocculation (adding a coagulating agent to create flocs, i.e., clusters of suspended matter that then settle due to their weight and are eliminated in the sludge.), as well as chemical processes such as ozonation.
Upon contact with ozone, the water undergoes an oxidation process that degrades numerous micro-pollutants (pesticides, residues of medications, etc.) and inactivates bacteria, viruses, and parasites.
This is followed by final steps of biological filtration on granular activated carbon and physical filtration (activated carbon filtration and membrane ultrafiltration) to remove remaining soluble pollution. Before being sent into the network, the water undergoes quality controls and chlorination, ensuring a lasting disinfectant effect to prevent the deterioration of the obtained water during distribution.
In recent years, Windhoek’s wastewater treatment plant has welcomed intrigued and interested visitors from Australia, Germany, the United Arab Emirates, and more. This is because the techniques developed in Namibia remain interesting for several reasons.
A less costly solution than desalination
For countries seeking new sources of drinking water, wastewater recycling is less energy-intensive and more environmentally friendly than seawater desalination, a more widespread technique worldwide. Wastewater purification consumes between 1 and 1.5 kWh per cubic meter, while desalination requires between 3 and 4 kWh per cubic meter. Additionally, the latter technique produces cumbersome waste: concentrate, containing salts and pollutants that are often discharged directly into the seas and oceans, disturbing ecosystems.
Despite all these advantages and the successful results of the Namibian case, wastewater purification is still in its infancy on a global scale because its implementation involves overcoming various barriers. Firstly, there is the cost of installation. Currently, only developed countries have been able to finance such projects, either domestically (in the United States, Singapore, etc.) or abroad. For example, behind the modernization of the Namibian plant is a public-private partnership involving the French company Veolia, the Australian-Indian company Wabag, and the city of Berlin.
Financial, legislative, and psychological barriers
Secondly, the legislation in different countries remains highly restrictive. In Europe, such a plant would currently not be authorized, and the only ongoing project for the purification of treated wastewater, the Jourdain Program in Vendée, will discharge water into a reservoir used as a reserve for drinking water production, not directly into the water distribution system: this is indirect purification.
Even when funds and laws are in place to allow the direct use of drinking water from treated wastewater, another significant barrier remains: making it acceptable to the population to drink treated wastewater and overcoming what is known as the “yuck factor”. In 2000, a wastewater purification plant in a neighborhood in Los Angeles, which cost $55 million to build, had to close a few days after its opening because “never make people drink toilet water” had become an election promise for a politician vying for the mayor’s seat.
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In Namibia, such a problem did not arise during the introduction of wastewater purification because the residents of Windhoek, then under apartheid rule, were confronted with the reality three months after the first plant went into operation. However, in a November 1968 article for the Sunday Tribune, the journalist covering the announcement of this new wastewater recycling reports that the mayor of Windhoek at the time, in a blind taste test, preferred the taste of treated wastewater over that of water from conventional sources.
The example of Singapore
However, not informing the population in advance is a radical and ill-advised solution because raising awareness among people about the scarcity of water resources and the need for more sustainable alternatives remains the best way to launch such a project. This is what allowed the success of Singapore’s project, which focused heavily on communication about the project of purifying treated wastewater, organizing visits to the treatment plant, or showing the then Prime Minister calmly drinking a glass of this new recycled wastewater.
Thus, the “yuck factor” transformed into a national pride. Pride in mastering cutting-edge technology and pride in gaining more independence from neighboring Malaysia, which remained its main supplier of drinking water and with whom diplomatic relations could be tense.
Beware of the “rebound effect”
But to strive for water self-sufficiency, Singapore did not put all its eggs in one basket and also invested in seawater desalination, rainwater harvesting, and reducing its residents’ water consumption (from 165 L per day per capita in 2000 to 141 L in 2018).
For all proponents of better water resource management, this frugality is crucial, both upstream and alongside the development of wastewater purification projects, in order to combat what is now known as the “rebound effect”. This expression describes the unencumbered and increasing use of water resources following the implementation of desalination or treated wastewater reuse techniques. These new water resources should only be regarded as a means to meet existing and essential needs and uses, often in substitution for drinking water, and not as an invitation to create new ones.
To maximize the resources at our disposal, the wastewater treatment plants of the future should also value the waste produced by the wastewater treatment processes, transforming phosphorus and nitrogen into nutrients useful for agriculture, or producing energy through methane digestion using the waste collected during treatment.
This article is part of a project involving The Conversation France and AFP Audio. It has received financial support from the European Journalism Centre, as part of the “Solutions Journalism Accelerator” program supported by the Bill and Melinda Gates Foundation. AFP and The Conversation France have maintained their editorial independence at every stage of the project.
Julie Mendret, Associate Professor, HDR, University of Montpellier
